Originally Posted by
cny-bikeman
Not quite following you, but with such a method any error made in the initial measurement will be multiplied by the ratio applied.
Keeping mind that we're measuring fork rake, not something that needs to be measured to within 0.001mm.
With a minimal amount of diligence, this method will be accurate to within 1-2mm.
So, I wish I could draw, but again you're running a string from the center of the axle to the top of the steerer. Then measuring at a right angle from the steerer to the string at specific distance along the way. That gives you the sign of the angle (opposite /hypotenuse). Now measure the total length of the string,top to axle, and apply the rule for similar triangles and add the radius of the steerer to get the rake.
You're right that whatever error you made on the first measurement will be multiplied, but other methods also have errors, including a large one if the fork isn't perfectly squared up when rotated.
In any case, I don't claim this is THE way to measure rake, or even the best way. I was just offering another method as an option for those interested.